Red, green and blue (RGB) lasers offer demonstrable benefits over incandescent light sources for high-performance imaging applications. Greater color saturation, contrast, sharpness, and color-gamut are among the most compelling attributes distinguishing laser displays from conventional imaging systems employing arc lamps. In spite of these performance advantages, however, market acceptance of laser display technology remains hindered as a result of its higher cost, lower reliability, larger package size and greater power consumption when compared to an equivalent lumen output lamp-driven display.
To compare laser projection technology with conventional technologies, it is instructive to examine two fundamental parameters which relate to their ultimate practicality. The first parameter can be defined as optical efficiency—in this case, the lumens of output per watt of input to the light source. The second is cost compatibility, that is, the extent to which the technology in question yields a cost effective solution to the requirements of a specific application.
Based on these parameters, a red/green/blue (RGB) semiconductor/microlaser system, consisting of three lasers or laser arrays, each operating at a fundamental color, appears to be the most efficient, high brightness, white light projection source for display applications to date. Semiconductor laser operation has been achieved from the UV to the IR range of the spectrum, using device structures based on InGaAlN, InGaAlP and InGaAlAs material systems. Desirable center wavelength ranges are 610–635 nm for red, 525–540 nm for green, and 445–470 nm for blue, as discussed below. An optical source with this spectrum provides a greater color gamut than a conventional arc lamp approach and projection technology which uses blackbody radiation.
Laser radiation is inherently narrow band and gives rise to the perception of fully-saturated colors. Unfortunately, narrow band light incident on random rough surfaces (such as a projection screen) also introduces an unacceptable image artifact known as “speckle”. The visual effects of speckle detract from the aesthetic quality of an image and also result in a reduction of image resolution. Consequently, in the context of high resolution display systems, it is generally deemed essential that speckle be eliminated. A variety of “de-speckling” techniques can be used to reduce this artifact to “acceptable levels”, but only at the expense of a further loss in efficiency, which negatively impacts cost, reliability, package size, and power consumption.
Known speckle reduction techniques tend to disturb the spatial or temporal coherence of laser beams through optical path randomization and/or spectral broadening. However, most of these solutions are expensive and technically complex, relying, for example, on mode-locking techniques to produce very short pulses in the order of 1 ps to increase the optical bandwidth. Ideally, the spectral bandwidth for a projection display light source should be on the order of several nanometers (i.e., 5–15 nm). Such a light source could be considered quasi-monochromatic—sufficiently broadband for the cancellation of speckle yet sufficiently narrow band for color purity. There is simply no laser-based RGB light source in existence with these properties.
It would therefore be desirable to provide a laser-based RGB light source with a spectral width of approximately 5–15 nm at the fundamental RGB wavelengths (approximately 620 nm for red, 530 for green, and 460 nm for blue), that is compact, efficient, reliable and inexpensive to manufacture. The practical purpose of the RGB laser-based light source is to achieve a high brightness (>1000 lumens) projection display on a screen of approximately 7.5 feet diagonal with a reduction in speckle. The demand for an RGB laser light source having several nanometers of bandwidth is universal for the vast majority of projectors (i.e., LCoS, p-Si LCD, DLP, and possibly GLV).